No transparency on imported planes
Although you didn't specify, it sounds like you are using the Add Images As Planes add-on.



You'll want to pay attention to the Operator Panel options that appear while the file browser window is open.

These options are contextual and vary depending on whether you are using Blender Internal or Cycles.

For Cycles it will look like this:



Be sure to select either Diffuse &amp; Transparent or Emission &amp; Transparent. With the former, scene lighting will affect the image texture. With the latter it will be shadeless.

You will also have to check the Use Alpha option. You can select either Premultiplied or Straight alpha, depending on which schema your file uses for its transparency.